Laminated structures constructed from adhesively joined sheet material layers

ABSTRACT

One or more sheet material layers in a multi-layer laminated structure is reinforced by adhesive bridges that extend through openings in the layer and tie together the layers on either side of the reinforced layer. A plurality of openings are provided in the layer or layers to be reinforced. The openings are spaced apart and distributed over the surface of the layer. Accordingly, when the layer is incorporated into a multi-layer structure, the adhesive applied to the layer, or to the layers on either side thereof, is forced through the openings so that a continuous adhesive connection is formed between the layers on opposite sides of the reinforced layer via the adhesive bridges that extend through the openings.

FIELD OF THE INVENTION

[0001] The present invention relates to structures made from multiplelayers of sheet material adhered together. The invention relates moreparticularly to such structures in which the various layers are ofdissimilar materials or have different strength properties.

BACKGROUND OF THE INVENTION

[0002] A variety of products are constructed from multiple layers ofsheet material joined together by adhesive, referred to herein aslaminated structures. Laminated structures can be formed of manydifferent types of rigid or flexible sheet materials. The motivation formaking a structure as a laminated structure, as opposed to using asingle layer of equivalent thickness, can vary depending on theparticular application. For instance, in the case of a laminated woodstructure such as plywood, the direction in which the grain runs in thevarious layers can be alternated between 0° and 90° directions so thatthe resulting plywood has similar bending strength in both directions.In contrast, a single wood layer of the same type and thickness wouldhave a significantly lower bending strength in one direction than theother because of the effect of the grain.

[0003] In other cases, multiple layers are used because the particularmaterial of which the structure is to be made is available only insheets whose thickness is substantially smaller than the neededthickness of the structure. For instance, many types of paperboardstructures are formed as laminated structures because paperboardgenerally is not available in thicknesses greater than about onemillimeter, whereas the structure to be formed may have to have athickness of several millimeters or more to meet strength and/ordimensional requirements. As an example, paperboard tubes aremanufactured for various uses, including as containers for products, ascores for winding paper, film, and textiles, as concrete forms, asstructural members, and others. In many of these types of tubes,dimensional and/or strength requirements dictate that from three to asmany as 25 or more layers of paperboard must be used to build the tube.A paperboard tube typically is made by sequentially wrapping a pluralityof paperboard plies about a mandrel having the desired shape of thetube. Adhesive is applied to the plies to join them together.

[0004] The strength of a paperboard tube depends on a number of factors,and there are several different strength properties one or more of whichmay be more important than others in a particular application. Animportant strength property that is assessed and taken into account inthe design of paperboard tubes by the assignee of the presentapplication, particularly with respect to paperboard winding cores, isthe flat crush strength of the tube. In a flat crush test, the tube isloaded between two flat plates parallel to the axis of the tube. Oneplate is held stationary and the other plate is moved toward thestationary plate at a defined slow rate, and the load exerted on thetube is continuously monitored. The flat crush strength of the tube isthe highest measured load before the tube is crushed.

[0005] Another significant strength property of a paperboard tube is theradial crush strength. Radially inward pressure loads can be exerted ontubes in some applications, such as when a web material is wound tightlyabout a paperboard winding core. Such winding cores must be able towithstand the expected radial crush loads without failing.

[0006] The beam bending strength of paperboard tubes can also be animportant strength parameter. In the case of a winding core, forinstance, the core is usually supported at its ends and the substantialweight of a roll of web material wound on the core must be toleratedwithout the core failing in bending.

[0007] The axial strength of a paperboard tube can also be an importantstrength parameter in some cases. Yet another important strengthproperty of paperboard tubes in some applications is the tube's abilityto withstand very high-speed rotation about its axis without failing asa result of radially outward centrifugal forces.

[0008] The various strength properties of a paperboard tube depend to alarge extent on the strength properties of the paperboard plies makingup the tube. In general, a paperboard tube of a given diameter and wallthickness can be increased in strength by making the tube frompaperboard plies of a higher strength. Paperboard materials of variousgrades are commercially available. The grade of a paperboard isgenerally understood in the industry to correlate with the strength ofthe paperboard.

[0009] Laminated structures such as those described above tend to belimited in strength by the strength of the weakest link in thestructure. In many laminated structures, the sheet material layers areweaker than the adhesive that binds them together. Where the structureis made up of layers of dissimilar materials, and hence differentstrengths, the factor limiting the strength of the structure thereforetends to be the strength of the weakest sheet material layer.Nevertheless, there may be good reasons to use such weaker layers in thestructure, as opposed to using all strong layers. For example, as noted,in some cases there may be a cost advantage to using at least someweaker layers in the structure. In other cases, the weaker layer mayserve another purpose that is needed and cannot be fulfilled by theother stronger layers; as an example, the weaker layer may be includedbecause it serves as a needed fluid barrier while the other strongerlayers do not. It would be advantageous to be able to reinforce suchweaker layers to improve the strength of the laminated structure.

[0010] In some laminated structures constructed from layers ofdissimilar materials, some layers may not be as readily bondable to theadhesive used for joining the layers together as other layers of thestructure. As a result, the weak link in the structure may be theadhesive bond between such a less-bondable layer and its adjacent layeror layers. It would be desirable to be able to eliminate this weak link.

[0011] In other laminated paperboard structures, the chosen paperboardmaterial for constructing the structure may be such that it does notbond to the adhesive as well as would be desired for optimal strength.For instance, paperboards that are densified to increase their strengthsometimes tend to have poorer adhesive bonding than paperboards of lowerdensity and strength. The strength benefit that such stronger pliesprovide thus can be partially offset by the lower adhesive bond strengthbetween the plies. It would be desirable to remedy this situation.

SUMMARY OF THE INVENTION

[0012] The present invention addresses the above needs and achievesother advantages. A laminated structure in accordance with a firstaspect of the invention comprises a plurality of sheet material layersjoined together by an adhesive, the sheet material layers including atleast one intermediate layer located other than on an outer surface ofthe laminated structure (i.e., located between other outwardly disposedlayers). The intermediate layer preferably is formed of a materialhaving a lower modulus or strength than that of the outwardly disposedlayers. In a conventional laminated structure the anchoring of theoutwardly disposed layers to each other occurs through the intermediatelayer; thus, if the intermediate layer is weak, the overall strength ofthe structure is comprised. In accordance with the invention, however,the intermediate layer has a plurality of openings formed thereinthrough which the adhesive penetrates so as to form adhesive bridgesthat extend through the intermediate layer and anchor the outwardlydisposed layers to each other. Thus, the adhesive bridges reinforce theweaker intermediate layer and augment the anchoring of the outwardlydisposed layers to each other.

[0013] The invention is applicable to various types of laminatedstructures. The sheet material layers can be formed of any suitablematerial, and can be rigid in some cases while in other cases they maybe flexible.

[0014] In accordance with another aspect of the invention, the laminatedstructure comprises a paperboard structure constructed of a plurality ofpaperboard plies one or more of which is reinforced with the adhesivebridges. The paperboard plies can all be identical or can be ofdifferent paperboard materials. Preferably, the openings in thereinforced ply collectively have a total area making up about 2 to 25percent of the surface area of the ply, more preferably about 5 to 20percent of the ply surface area. Each opening can have an area fromabout 0.1 mm² to about 20 mm², more preferably about 1 mm² to about 15mm².

[0015] Various sizes and shapes of openings can be formed through theply or plies to be reinforced, such as circular holes, polygonal holes,slits, etc. In preferred embodiments of the invention, the openingscomprise elongate slits (i.e., having a length dimension substantiallygreater than the width dimension) so that the perimeter of the openingis increased for a given opening area, or alternatively the opening areais reduced for a given perimeter. For instance, the openings cancomprise rectangular holes having a length several times greater thanthe width. The openings preferably are oriented so that their lengthdirections are substantially aligned with the direction along which thelargest tensile loads are expected to be placed on the paperboard pliesduring a particular application. In this manner, the openings result ina relatively smaller reduction of the cross-sectional area of the plythat is available to support the tensile loads, compared to alternativeorientations of the openings. It is further beneficial to stagger theopenings in the lengthwise direction of a ply so as to limit the numberof openings aligned along any given line extending in the widthwisedirection of the ply; this also helps maximize the cross-sectional areaof the ply available to support the tensile loads.

[0016] The openings in a ply or plies can be provided in various ways.The paperboard could be produced on a paper machine such that thepaperboard comes off the machine having a substantial porosity or havingsmall holes such that adhesive can penetrate through it. The openingsalternatively can be made by pricking or puncturing a formed ply with asharp tool or punch die.

[0017] Each ply having the openings preferably is disposed between twoother plies, which may or may not have openings. Preferably, the pliesforming the outer surfaces of the laminated structure do not haveopenings.

[0018] In some preferred embodiments, plies having openings arealternated with plies not having openings. In other embodiments, aplurality of contiguous plies each having openings can be incorporatedin the laminated structure. Where two contiguous plies both haveopenings, it is considered preferable but not essential to stagger theopenings so that the openings in one ply are not aligned with theopenings in the adjacent ply.

[0019] The invention is applicable to multi-grade paperboard structuressuch as paperboard tubes. In particular, one or more lower-strengthpaperboard plies can be incorporated into a tube and can be reinforcedby openings with adhesive bridges. Thus, multi-grade paperboard tubesformed of higher-strength and lower-strength paperboard plies can beimproved in strength, relative to equivalent tubes not having openingsthrough the lower-strength plies. In preferred embodiments of theinvention, about 5 to 95 percent, more preferably about 30 to 70percent, of the wall thickness of a multi-grade paperboard tube is madeup of lower-strength paperboard plies, and some or all of thelower-strength plies have openings therethrough. The lower-strengthplies having openings can be alternated with higher-strength plies,which may or may not have openings; alternatively, a plurality ofcontiguous lower-strength plies having openings can be included in thetube.

[0020] When a lower-strength ply is provided with openings and adhesivebridges are formed through the ply linking together stronger plies oneither side of the weak ply, the resulting structure's strength is nolonger limited by that of the weak ply, or at least is limited to alesser extent than it otherwise would be.

[0021] The openings and adhesive bridges can also be advantageous inpaperboard structures formed of densified high-strength plies, or formedwith treated (e.g., saturated or coated) paperboard, which tend to bondwith adhesive less tenaciously than may be desired. The adhesive bridgescan be provided through the plies to form a continuous adhesive lacingor webbing to provide added strength to the ply interfaces.

BRIEF DESCRIPTION OF THE DRAWINGS

[0022] The above and other objects, features, and advantages of theinvention will become more apparent from the following description ofcertain preferred embodiments thereof, when taken in conjunction withthe accompanying drawings in which:

[0023]FIG. 1 is a perspective view of a paperboard tube in accordancewith the invention;

[0024]FIG. 2 is an enlarged cross-sectional view taken through the wallof the tube along line 2-2 in FIG. 1;

[0025]FIG. 3 is a plan view of a paperboard ply having openingstherethrough in accordance with the invention;

[0026]FIG. 4 is a view similar to FIG. 2, showing a five-ply paperboardstructure in accordance with another embodiment of the invention;

[0027]FIG. 5 shows another five-ply paperboard structure in accordancewith yet another embodiment of the invention;

[0028]FIG. 6 is a schematic depiction of a paperboard tube being formedon a mandrel in accordance with the invention; and

[0029]FIG. 7 is a plan view of a paperboard strip having an alternativeconfiguration of openings formed as elongate slits.

DETAILED DESCRIPTION OF THE INVENTION

[0030] The present invention now will be described more fullyhereinafter with reference to the accompanying drawings, in whichpreferred embodiments of the invention are shown. This invention may,however, be embodied in many different forms and should not be construedas limited to the embodiments set forth herein; rather, theseembodiments are provided so that this disclosure will be thorough andcomplete, and will fully convey the scope of the invention to thoseskilled in the art. Like numbers refer to like elements throughout.

[0031] The present invention is applicable to various multi-layerpaperboard structures formed by adhering multiple layers or plies ofpaperboard to one another. An example of such a product is a paperboardtube 10 as shown in FIG. 1. The tube 10 comprises a plurality of stripsor plies of paperboard wrapped or wound one atop another about an axisand adhered together. In its simplest form in accordance with theinvention, the tube 10 is formed of three plies. FIG. 2 shows across-section through the body wall 12 of such a three-ply tube. Thebody wall 12 comprises paperboard plies 14, 16, and 18. Adhesive 20 isapplied between the opposing faces of the neighboring plies to adherethe plies together. In accordance with the invention, the intermediateply 16 includes holes or openings 22 extending through it. Accordingly,when the adhesive 20 is applied to the plies in a fluid state, theadhesive intrudes into the openings 22. When the adhesive solidifies,substantially solid bridges of adhesive 20 extend through the openings22, thus tying together the plies 14 and 18 on either side of theintermediate ply 16. In other words, a continuous adhesive connection isformed between the plies 14, 18.

[0032]FIG. 3 shows a plan view of the ply 16 having the openings 22. Theopenings 22 are spaced apart and distributed over the surface of theply. Various shapes of openings 22 can be used in accordance with theinvention. As shown, the ply 16 has punched circular openings 22, butother shapes can be used instead, including but not limited to polygonalholes, slits, etc. The openings can be formed in various ways, such asby punching, slitting, piercing, etc.

[0033] The sizes of the openings 22 and the spacing between them can bevaried. Preferably, the openings 22 collectively have a total area thatconstitutes from about 2 to 25 percent of the surface area of thepaperboard ply, and more preferably about 5 to 20 percent of the surfacearea. Each opening 22 preferably has an area from about 0.1 mm² to about20 mm², more preferably about 1 mm² to about 15 mm². However, theinvention in its broadest aspects is not limited to any particular sizesof openings, except that it should be noted that the openings inaccordance with the invention are much larger than the microscopic poresthat are inherently present in paperboard. Such pores do not allowadhesive to penetrate through them during a normal manufacturing processof a paperboard structure, whereas the openings in the paperboard plyformed in accordance with the invention do allow adhesive to readilypenetrate through them to form adhesive bridges.

[0034] An alternative paperboard structure 30 in accordance with theinvention is depicted in FIG. 4. The structure 30 has five paperboardplies 32, 34, 36, 38, 40. Each of the three intermediate plies 34, 36,38 includes openings 22 therethrough.

[0035] Yet another paperboard structure 50 in accordance with theinvention is shown in FIG. 5. The structure 50 also has five paperboardplies 52, 54, 56, 58, 60. Alternate plies have openings; thus, the plies54 and 58 have openings 22 but the remaining plies do not.

[0036] Structures such as those depicted in FIGS. 1-5 can be formed asmulti-grade structures. Thus, for instance, in the structure of FIG. 2,the intermediate ply 16 having the openings 22 can be a relativelylow-strength grade of paperboard while the two exterior plies 14, 18 canbe formed of a relatively higher-strength grade of paperboard. Theadhesive bridges extending through the openings 22 in the low-strengthply 16 can reinforce the ply so that it is no longer the weak link inthe structure. Instead, the limiting factor determining the strength ofthe structure can be the higher-strength plies on either side of thelow-strength ply.

[0037] A series of bench tests was performed on flat three-plypaperboard samples having the ordered construction H/L/H, where “H”signifies a ply of relatively higher grade and strength and “L”signifies a ply of relatively lower grade and strength. Thehigher-strength paperboard had a measured Scott Bond strength of 374J/m². As known in the art, the Scott Bond test subjects a paperboardsample to impacts in a direction tending to peel or split the sampleinto two thicknesses; the test is a measure of the Z-direction (i.e.,perpendicular to the plane of the sample) tensile strength of thesample.

[0038] Three different hole configurations were formed in the middle “L”plies: (1) no holes, (2) 3 mm diameter holes making up 8.8% of thesurface area of the ply, and (3) 4 mm diameter holes making up 15.6% ofthe ply surface area. Thus, nine different sample configurations weremade corresponding to the three different types of low-strength “L”plies and three different hole configurations. The plies were adheredtogether with dextrine adhesive. The samples were tested to determinetheir Scott bond strength. Three repeats were done for each sampleconfiguration, for a total of 27 tests. The results of the tests aregiven in Table I below: TABLE I Scott Bond Hole Dia Open Area Strength(J/m²) Average Increase L Ply (mm) % 1 2 3 (J/m²) % #1 0 0 221 242 242235 3 8.8 347 336 357 347 47.8 4 15.6 378 494 357 410 74.6 #2 0 0 189179 179 182 3 8.8 315 357 305 326 78.8 4 15.6 441 452 399 431 136.5 #3 00 231 252 242 242 3 8.8 368 357 441 389 60.9 4 15.6 368 315 305 329 36.2

[0039] For comparison purposes, the Scott bond strength of thehigher-strength paperboard was measured to be 374 J/m². Thus, it can beseen that with holes of 3 mm diameter, the average tested Scott bondstrength of the samples having the #3 low-strength ply is very close tothe Scott bond strength of the high-strength paperboard; without theholes, the strength is substantially lower than that of thehigh-strength paperboard. It is believed that without the holes, thestrength of the sample is limited by the strength of the weak ply, whichis the weak link where the sample fails in the test. With the holesproviding adhesive bridges between the two strong plies, however, theweak ply is no longer the weak link in the structure. Accordingly, thestrength of the structure ought to be closer to that of the strongplies. The test results in Table I support this notion.

[0040] Furthermore, with holes of 4 mm size, the average Scott bondstrength of the samples made with #1 and #2 low-strength plies actuallyexceeded the strength of the high-strength paperboard alone. Thestrength of the samples made with the #3 low-strength paperboard waslower than that of the high-strength paperboard, but air bubbles wereobserved in the adhesive in the holes in those samples; the bubbles arebelieved to have compromised the effect of the adhesive bridges.

[0041] Thus, the test results show that the strength of a multi-gradepaperboard structure can be significantly enhanced when the structure isconstructed in accordance with the present invention.

[0042] To confirm the results of the first series of tests, and tofurther investigate other variables that may affect the performance of amulti-layer paperboard structure made in accordance with the invention,a second series of tests was performed. Samples of multi-layerpaperboard structures were made in accordance with the followingconfigurations designated “A” through “D”:

[0043]A=H/L/H. Reference samples without openings, samples with 3 mmholes in the middle low-strength ply, and samples with 4 mm holes in themiddle low-strength ply were made and tested.

[0044]B=H/L_(0.5)/L_(0.5)/H, where L_(0.5) denotes a low-strength ply of0.5 mm thickness. Reference samples without openings and samples with 3mm holes in each of the middle low-strength plies were made and tested.The holes in the two middle plies were staggered to avoid superpositionof the holes.

[0045]C=H/L_(0.8)/L_(0.8)/H (same as B, except the middle plies arelow-strength plies of 0.8 mm thickness).

[0046]D=H/H/H. Reference samples without openings, samples with 3 mmholes in the middle low-strength ply, and samples with 4 mm holes in themiddle low-strength ply were made and tested.

[0047] A Scott Bond was performed for each of configurations A throughD, and the results are tabulated in Table II below: TABLE II Hole DiaOpen Scott Bond Increase Configuration (mm) Area % (J/m²) % A 0 0 189 38.8 290 53 4 15.6 399 111 B 0 0 280 3 8.8 403 44 4 15.6 498 78 C 0 0 3093 8.8 445 44 4 15.6 595 92 D 0 0 511 3 8.8 477 −6.6 4 15.6 431 −16

[0048] It can be seen that the construction of a paperboard structure inaccordance with the invention substatially improved the Scott Bondstrength. Increasing the hole diameter from 3 mm to 4 mm increased theScott Bond strength.

[0049] The results of the testing of configurations B and C show thatthe invention offers advantages even when the low-strength plies havingthe openings are not located contiguously between two high-strengthplies. In this case, the adhesive bridges through each low-strength plyare not made between two high-strength plies, but between onehigh-strength ply and one low-strength ply. The thickness of thelow-strength plies in this type of structure does not seem to have asignificant effect on the strength properties, as can be seen bycomparing the test results of configuration B with those ofconfiguration C in Table II.

[0050] During the course of the testing, it was observed that theadhesion between the adhesive bridges and the holes was mainly at theedges of the holes. Accordingly, it was hypothesized that the perimeterof each opening may be a more-significant factor than the total area ofthe opening, in terms of the impact on the strength properties of aresulting structure. If this hypothesis were true, a circle would be theworst shape of opening because it has the smallest perimeter for a givenarea. It would be preferable to use openings having a larger ratio ofperimeter to area. This could also have advantages in terms of the totalamount of adhesive added to the structure as a result of the openings.More specifically, for a given total amount of perimeter represented bythe sum of all of the openings, the total area of the openings could bereduced through appropriate selection of the shapes of the openings, sothat less adhesive is present in the openings. This would also have anadvantage in terms of the moisture add-on caused by the adhesive in theopenings.

[0051] To test this hypothesis, samples of the construction H/L/H havingan alternative shape for the openings in the middle low-strength plywere made and tested for Scott Bond strength. The openings wererectangular slits having a length of 5 mm and a width of 1.4 mm, eachslit thus having an area substantially equal to that of a 3 mm circularhole. The slit, however, had a perimeter of 12.8 mm, versus 9.4 mm forthe circular hole. All samples had the same number of openings and hencesame total open area; the slits provided a 36 percent larger totalperimeter than the holes.

[0052] It was suspected that the orientation of the slits in relation tothe direction of loading in the Scott Bond test might affect theresults, so two different orientations were tested. One batch of sampleshad the slits oriented so that their lengthwise directions were parallelto the direction of movement of the Scott Bond pendulum (i.e., parallelto the direction of propagation of the break); another batch had thelengthwise directions of the slits perpendicular to the direction ofmovement of the Scott Bond pendulum (perpendicular to the direction ofpropagation of the break).

[0053] Yet another variable that was investigated is the amount ofpressure applied to the plies to squeeze them together after applicationof the adhesive to the plies. Two different pressure levels were tested,5 kPa and 10 kPa.

[0054] The results of the Scott Bond tests for this series are tabulatedin Table III below: TABLE III Scott Bond Strength (J/m²) ConfigurationPressure = 5 kPa Pressure = 10 kPa Reference (no openings) 286 267 3 mmcircular holes 494 448 5 × 1.4 mm slits—parallel to 555 464 loadingdirection 5 × 1.4 mm slits—perpendicular 519 401 to loading direction

[0055] It will be noted that the slits provided a greater Scott Bondstrength enhancement than the circular holes when the slits wereoriented with their lengthwise directions parallel to the direction ofpropagation of the break. Accordingly, it should be possible to obtainthe same Scott Bond strength as that attained with the circular holes,using slits having a smaller total area. This should result in a loweradhesive usage for a similar strength performance.

[0056] The test results also show that the samples pressed with a higherpressure to bond the plies together had a lower Scott Bond strength thanthe samples pressed with a lower pressure. It is theorized that thehigher pressure reduced the volume of the adhesive available between theplies and in the open areas, and then drying of the adhesive causedshrinkage of the adhesive so that there may have been an inadequateamount to form adhesive bridges completely filling the open areas.Accordingly, it is believed that it may be advantageous to use anadhesive having a high solids content so as to reduce the shrinkage ofthe adhesive upon drying. The combination of smaller total open areaprovided by the slits and higher solids content of the adhesive shouldreduce the total moisture add-on by a significant amount.

[0057] In a multi-grade paperboard tube, it is theorized that thebenefit of the openings in the low-strength plies noted in theabove-summarized tests should be reflected in the strength of the tube;more particularly, a tube made in accordance with the invention shouldhave an improved strength relative to an otherwise identical tube nothaving openings for forming adhesive bridges.

[0058] Various configurations of multi-grade tubes are possible inaccordance with the invention. As shown in FIG. 5, for instance, aplurality of lower-strength plies can be included in a tube, each onehaving openings, and each being non-contiguous with the otherlower-strength plies; alternatively, the tube can include contiguouslow-strength plies having openings, as in the embodiment of FIG. 4.Although FIGS. 4 and 5 show five-ply structures, the invention is notlimited to any particular number of plies, and paperboard tubes with asmany as 25 or more plies can be made.

[0059] A multi-grade tube in accordance with the invention can includevarious numbers of high-strength and reinforced low-strength plies. Asfew as one low-strength ply can be included in a tube having from 3 to25 or more plies, such that the reinforced low-strength ply may compriseas little as 5 percent of the total wall thickness of the tube. On theother hand, a majority of the plies in a tube having 5 or more plies canbe low-strength plies reinforced in accordance with the invention, suchthat the low-strength plies may comprise as much as 95 percent of thebody wall thickness. More commonly, however, it is expected that thelow-strength plies will generally comprise from about 30 percent to 70percent of the body wall thickness.

[0060] It should also be noted that although the foregoing descriptiongives exemplary multi-grade tube constructions in which thelower-strength plies have the openings for forming adhesive bridges, itis also possible within the scope of the present invention to provideopenings in higher-strength plies.

[0061]FIG. 6 depicts an apparatus for making a paperboard tube inaccordance with the invention. The apparatus includes a cylindricalmandrel 70 on which the tube is formed. A plurality of paperboard plies72, 74, 76, . . . 78 are drawn from supply rolls and advanced to themandrel and are spirally wrapped onto the mandrel in overlappingarrangement with one another to form the tube. A winding belt (notshown) engages the paperboard tube on the mandrel and rotates the tubesuch that the tube advances along the mandrel in a screw fashion. Theplies are secured together by adhesive applied to various ones of theplies by adhesive applicators 80. In accordance with the invention, oneor more of the plies (in the illustrated embodiment, the ply 74) isprovided with openings distributed over its surface. The ply 74 can beprocured from a vendor with the openings already formed in it;alternatively, an in-line unit 90 can be provided for piercing,punching, or otherwise forming the openings in the ply drawn from itssupply roll, as schematically illustrated in FIG. 6.

[0062] Based on the tests summarized above, a preferred pattern ofopenings in a continuous paperboard strip used in forming spirally woundtubes is shown in FIG. 7. The paperboard strip 100 includes slits 102arranged in five rows that extend in the lengthwise direction of thestrip. The slits 102 are rectangular having a length dimension thatsubstantially exceeds the width dimension. The length dimensions of theslits are preferably aligned along the lengthwise direction of thestrip. Preferably, as shown, the slits are staggered so that any lineperpendicular to the lengthwise direction of the strip, such as theillustrated line L, intersects fewer than five slits (i.e., fewer thanthe number of rows of slits). Thus, in the illustrated strip 100, themaximum number of slits 102 that can be intersected by a lineperpendicular to the length of the strip is three, whereas there arefive rows of slits. This results in less reduction of thecross-sectional area of the paperboard available to support tensileloads in the lengthwise direction of the strip. When the paperboardstrip 100 is spirally wound during tube formation, the lengthwisedirections of the slits 102 are aligned along the direction of spiralwinding of the strip. Alternatively, the slits could be oriented withtheir lengthwise directions inclined to the lengthwise direction of thestrip by an angle equal to the complement of the spiral wind angle atwhich the strip is wound when forming a tube. In this manner, the slitscan be oriented axially along the tube. As yet another alternative, theslits can be inclined at the spiral wind angle so that the slits areoriented in the circumferential direction once the strip is wound ontothe tube, or at any other desired angle.

[0063] Slits having shapes other than the illustrated rectangular slitscan also be used in accordance with the invention.

[0064] Many modifications and other embodiments of the invention willcome to mind to one skilled in the art to which this invention pertainshaving the benefit of the teachings presented in the foregoingdescriptions and the associated drawings. Therefore, it is to beunderstood that the invention is not to be limited to the specificembodiments disclosed and that modifications and other embodiments areintended to be included within the scope of the appended claims.Although specific terms are employed herein, they are used in a genericand descriptive sense only and not for purposes of limitation.

What is claimed is:
 1. A paperboard product, comprising: at least threepaperboard layers disposed one atop another and joined together byadhesive applied between opposing faces of the layers to form amulti-layer paperboard structure, an intermediate one of the paperboardlayers having a plurality of openings therethrough spaced anddistributed over the faces thereof, adhesive penetrating through theopenings so as to form adhesive bridges that extend through theintermediate layer and tie the paperboard layers on opposite sides ofthe intermediate layer to each other.
 2. The paperboard product of claim1, comprising a paperboard tube, the paperboard layers being wrappedabout an axis of the tube.
 3. The paperboard product of claim 1, whereinthe openings in the intermediate layer collectively have a total areathat comprises from about 2 percent to about 25 percent of the surfacearea of the intermediate layer.
 4. The paperboard product of claim 1,wherein the openings are each about 0.1 mm² to 20 mm² in area.
 5. Thepaperboard product of claim 1, wherein the openings are each about 1 mm²to 15 mm² in area.
 6. The paperboard product of claim 1, comprising atleast four paperboard layers such that there are at least twointermediate layers, each intermediate layer having openings withadhesive bridges extending therethrough.
 7. The paperboard product ofclaim 1, comprising paperboard layers formed of a relativelyhigher-strength grade of paperboard, and at least one paperboard layerformed of a lower-strength grade of paperboard and having openings withadhesive bridges extending therethrough to reinforce said lower-strengthpaperboard layer.
 8. The paperboard product of claim 7, wherein thehigher-strength paperboard layers and the at least one lower-strengthpaperboard layer are alternated in position with one another.
 9. Thepaperboard product of claim 7, wherein there are a plurality oflower-strength paperboard layers having openings with adhesive bridgesextending therethrough.
 10. The paperboard product of claim 1, whereinthere are at least five paperboard layers and every other paperboardlayer has openings with adhesive bridges extending therethrough.
 11. Thepaperboard product of claim 1, comprising a paperboard tube, wherein thepaperboard layers comprise continuous strips of paperboard each wound ina spiral winding direction about an axis of the tube, and wherein theopenings in the intermediate paperboard layer comprise slits having alengthwise dimension substantially exceeding a widthwise dimensionthereof, the slits being oriented with the lengthwise dimensionssubstantially aligned along the spiral winding direction of theintermediate paperboard layer.
 12. The paperboard product of claim 11,wherein the adhesive used for adhering the plies together is ahigh-solids-content adhesive such that the adhesive tends not tosubstantially shrink upon drying.
 13. The paperboard product of claim 1,wherein the openings in the intermediate paperboard layer comprise slitshaving a lengthwise dimension substantially exceeding a widthwisedimension thereof, the slits being arranged in a plurality m of parallelrows extending along a first direction of the intermediate paperboardlayer, and the slits being staggered such that the maximum number ofslits intersected by any line extending across the intermediatepaperboard layer perpendicular to the first direction is fewer than m.14. A multi-grade paperboard tube, comprising: a plurality of paperboardplies wrapped one upon another about an axis and joined together byadhesive applied between opposing faces of the plies so as to form amulti-layer paperboard tube structure, the plies including paperboardplies formed of a relatively higher-strength grade of paperboard, and apaperboard ply formed of a lower-strength grade of paperboard and havingopenings extending therethrough, said lower-strength paperboard plybeing disposed between two paperboard plies, and adhesive penetratingthrough the openings in the lower-strength paperboard ply so as to formadhesive bridges that extend through the lower-strength paperboard plyand tie the paperboard plies on opposite sides thereof to each other.15. The multi-grade paperboard tube of claim 14, wherein thelower-strength paperboard ply is contiguous with each of twohigher-strength paperboard plies on opposite sides thereof.
 16. Themulti-grade paperboard tube of claim 14, wherein there are 5 to 25 pliesin total.
 17. The multi-grade paperboard tube of claim 14, wherein thereare a plurality of lower-strength paperboard plies having openings. 18.The multi-grade paperboard tube of claim 17, wherein the lower-strengthpaperboard plies are non-contiguous with one another.
 19. Themulti-grade paperboard tube of claim 17, wherein the lower-strengthpaperboard plies collectively make up about 5 to 95 percent of a totalradial thickness of a body wall of the paperboard tube.
 20. Themulti-grade paperboard tube of claim 17, wherein the lower-strengthpaperboard plies collectively make up about 30 to 70 percent of a totalradial thickness of a body wall of the paperboard tube.
 21. Themulti-grade paperboard tube of claim 14, wherein the openings in thelower-strength paperboard ply collectively have a total area thatcomprises from about 2 percent to about 25 percent of the surface areaof the lower-strength paperboard ply.
 22. A paperboard tube, comprising:a plurality of paperboard plies wrapped one atop another about an axisand joined together by adhesive applied between opposing faces of theplies to form a paperboard tube structure, the plies including aninterior paperboard ply proximate an inner surface of the tubestructure, an exterior paperboard ply proximate an outer surface of thetube structure, and at least one intermediate paperboard ply positionedbetween the interior and exterior paperboard plies and having aplurality of openings extending therethrough, adhesive penetratingthrough the openings so as to form adhesive bridges that extend throughthe intermediate paperboard ply and tie together paperboard plies onopposite sides thereof.
 23. The paperboard tube of claim 22, whereinthere are a plurality of intermediate paperboard plies each havingopenings with adhesive bridges extending therethrough.
 24. Thepaperboard tube of claim 23, wherein all plies positioned between theinterior and exterior paperboard plies have openings with adhesivebridges extending therethrough.
 25. The paperboard tube of claim 22,wherein the openings in the intermediate paperboard ply comprise slitshaving a lengthwise dimension substantially exceeding a widthwisedimension thereof.
 27. The paperboard tube of claim 26, wherein theslits are oriented with the lengthwise dimensions thereof parallel to alengthwise direction of the intermediate paperboard ply.
 28. A laminatedstructure, comprising: a plurality of sheet material layers joinedtogether by an adhesive, the sheet material layers including at leastone intermediate layer sandwiched between a pair of outwardly disposedlayers, the intermediate layer being formed of a material having a lowermodulus than that of the outwardly disposed layers, and the intermediatelayer having a plurality of openings formed therein through which theadhesive penetrates so as to form adhesive bridges that extend throughthe intermediate layer and anchor the outwardly disposed layers to eachother.
 29. The laminated structure of claim 28, wherein the sheetmaterial layers are flexible.
 30. The laminated structure of claim 28,wherein the adhesive has a modulus greater than that of the sheetmaterial layers.
 31. The laminated structure of claim 28, wherein the atleast one intermediate layer has a relatively lower affinity to bondingwith the adhesive and the outwardly disposed layers have a relativelyhigher affinity to bonding with the adhesive .
 32. The laminatedstructure of claim 31, wherein the at least one intermediate layercomprises a polymer film.
 33. The laminated structure of claim 28,further comprising at least one additional intermediate layer betweenthe outwardly disposed layers.
 34. The laminated structure of claim 33,wherein all of the intermediate layers are of the same material.